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The
pseudo random (PN) sequence is a bit stream of ‘1’s and ‘0’s occurring
randomly, or almost randomly, with some unique properties. It is widely used in
direct sequence spread spectrum wireless communication systems, for example,
synchronous CDMA or asynchronous CDMA. The spread spectrum CDMA scheme is one
in which the transmitted signal is spread over a wide frequency band, much
wider than the minimum bandwidth required to transmit the information being
sent. In general, the spread spectrum signals are commonly used for-

·Combating or suppressing the detrimental
effects of interference due to jamming, interference arising from other users
of the channel, and self-interference due to multipath propagation;

·Hiding a signal by transmitting it at
low power density and, thus making it difficult for an unintended listener to
detect it in the presence of background noise;

·Achieving message privacy in the presence
of other listeners.

·Sharing the limited frequency resource
to satisfy the demand of the increasing capacity in the public network

The desired properties of PN sequence depend clearly
on the target application. Our goal is to study the code selecting in WCDMA
system, the main emphasis is on the control of the interference arising from
other users of the channel. The PN sequence selected should have high
auto-correlation and low cross-correlation among different users’ PN codes.

A block diagram of spreading and despreading signal in
baseband of CDMA is shown in Fig .

In
transmitter, the spreader consists of multiplying the input data by a pseudo
noise (PN) sequence, the bit rate of which is much higher than the data bit
rate. This increases the data rate while adding redundancy to the system. The
data bits modulated by spreading are called chips. The ratio of the chip
bit rate to the data bit rate is called the spreading factor (SF). The
resulting waveform is wideband, noise like signal .

Figure
5.1 Spreader Block Diagram

When the signal is received in the
receiver side, the spreading is removed from the desired signal by multiplying
with the same PN sequence that is exactly synchronised to the received chip
level signal. When despreading is applied to the interference generated by
other users’ signals, there is no despreading, this means that each spread
spectrum signal should behave as if it were uncorrelated with every other
spread signal using the same band. Therefore, CDMA codes are designed to have
very low cross-correlation.